The present invention relates to a lithography system in which intensities of individual beams from a multitude of beams are determined, comprising a measuring device with a sensor having a sensor area adapted for simultaneously sensing a plurality of beams and providing an aggregated signal thereof. The present invention further relates to a method for calculating such individual beam intensities dependent on the measured aggregated signal.
Exposure beams of charged particles, light and/or other types of radiation are being used in the industry, inter alia, in the manufacture of highly integrated and micro patterned semiconductor devices. Such semiconductor devices are usually formed on a semiconductor wafer on which layers of suitable materials are deposited, patterned and subsequently etched away according to a predetermined pattern. The exposure beams are often used in the patterning step. Portions of sacrificial material which are exposed to such a beam will be etched away in a following step, whereas portions that have not been exposed to a beam will remain, resulting in a pattern on for example a wafer.
In this process the dose of the exposure beams has to be accurately monitored and possibly adapted accordingly; if the dose is too low the sacrificial material will not react sufficiently and the sacrificial material will not be etched away in the following step, whereas if the dose is too high some part of the beam may spill over into areas which should not be exposed, resulting in a larger surface being etched away than desired. Both cases result in a pattern different from the intended pattern.
Unfavorably, the dosage of the beams reaching the sacrificial material is susceptible to changes over time due to for example changes in the beam source, alterations to the exposure system, changes in the pattern to be transferred, etc. There is a need therefore to be able to determine the dose of beams used in beam exposure systems. Preferably such a dose can be determined quickly for many beams, at or near the point where the exposure beam(s) hit the target.
For determining the dose of a charged particle (CP) beam or CP beams in a fast and reliable manner some solutions have already been proposed:
In JP2004-200549A a multiple-beam lithography system is described in which the absolute value of the total of the currents in all m×n matrix form electron beams is measured, by using a Faraday cup provided on a plane, and the relative value of each the m×n beams is measured using a semiconductor detector, or by using a combination of a scintillator and a photomultiplier, for the determination of reflected electrons or the secondary electrons. The relative current values, standardized for all the currents, are shown in a chart, and this enables at a glance the detection of anomalous values for improved reliability in the drawing motions to follow. The Faraday cup is used to accurately yet relatively slowly determine an absolute total current of either one or all electron beams which impinge thereon. The relative current measurement provides a relatively fast yet less accurate and relative representation of the current of one or more electron beams. From the relative currents for all individual electron beams estimates of the absolute currents for all individual electron beams can be made when the combined total current of all electron beams is known.
In US 2006/0,138,359 A1 ('359) a CP beam exposure apparatus is disclosed which splits a CP beam from a CP beam source into a plurality of CP beams by a plurality of apertures formed in an aperture array to execute exposure using the plurality of CP beams, the apparatus comprising:                a detection unit which detects an intensity of the CP beam passing through the aperture of the aperture array, and        a grid array which adjusts an intensity of the CP beam on the basis of the detection result obtained by said detection unit.In an embodiment according to '359 the intensity of the beam or beams is measured at two levels in the exposure apparatus. First, for each individual beam or group of beams passing through the aperture array the intensity is measured. This is done by blanking out all other beams passing through the aperture array and positioning a Faraday cup close to the individual CP beam to measure the beam or group of beams to be measured. The Faraday cup is placed on or near the wafer surface in order to obtain a measurement of the beam or group of beams close to the wafer surface. The measured intensity for each beam or group of beams is stored in a memory and is used later on as a reference value.        
Additional measurements of the beam intensity can be performed during the patterning phase of a target such as a wafer. The intensities of parts of the beam which are blocked by surfaces on the aperture array are measured, said surfaces being isolated from each other and associated with a single aperture or group of apertures. It is assumed that the intensity of a part of the CP beam being blocked by the surface surrounding the aperture is similar to that of a part of the CP beam which passes through the aperture. Thus it becomes possible to determine an approximate intensity of the plurality of CP beams which emerge from the aperture array based on the CP beam intensity measured on the associated blocking surfaces on the aperture array. The values obtained during these measurements are compared to the reference values and the system is adapted to maximize the uniformity of the CP beams emerging from the aperture array.
When using the above apparatus to measure actual individual beam intensities of beams emerging from the aperture array, the measuring device, in this case a Faraday cup, has to be brought into a position close to where the beam would reach the target during the patterning phase. This positioning of the measuring device has to be repeated for each beam out of the plurality of beams emerging from the aperture array, which takes time, and becomes especially impractical for systems in which thousands of beams have to be measured. Furthermore, the detecting surfaces on the aperture array which measure the approximate CP beam intensities at a point before reaching the level of a target have to be precisely aligned and are not allowed to interfere substantially with the CP beams. Precise alignment of the detecting surfaces necessitates a complex design. Moreover, when the apertures are very small, for instance to enable thousands of beams to emerge from the aperture array or to reach a high beam resolution, the CP beams may be influenced by electrical signals emitted by the detecting surfaces, making this approach impractical for systems in which thousands of individual CP beams are used.
Obviously there still is a need in the industry for a system and method for quickly determining the dose of each beam in a multitude (for example, tens of thousands) of beams which delays the manufacturing process for a minimum amount of time, preferably using only a single sensor.